1. Field of the Invention
The present invention relates to light sources, and more specifically, to a hybrid light source having a continuous-spectrum light source, a discrete-spectrum light source, and drive circuits for controlling the amount of power delivered to each of the light sources.
2. Description of the Related Art
From the dawn of mankind, the sun has proved to be a reliable source of illumination for humans on Earth. The sun is a black-body radiator, which means that it provides an essentially continuous spectrum of radiated light that includes wavelengths of light ranging across the full range of the visible spectrum. As the human eye has evolved over millennia, man has become accustomed to the continuous spectrum of visible light provided by the sun. When a continuous-spectrum light source, such as the sun, shines on an object, the human eye is able to perceive a wide range of colors from the visible spectrum. Accordingly, continuous-spectrum light sources (i.e., black-body radiators) provide a more pleasing and accurate visual experience for a human observer.
The invention of the incandescent light bulb introduced to mankind an artificial light source that approximates the light output of a black-body radiator. Incandescent lamps operate by conducting electrical current through a filament, which produces heat and thus emits light. Since incandescent lamps (including halogen lamps) generate a continuous spectrum of light, these lamps are often considered continuous-spectrum light sources. FIG. 1A is a simplified graph showing a portion of the continuous spectrum SPCONT of a halogen lamp, which ranges across the visible spectrum from a wavelength of approximately 380 nm to a wavelength of approximately 780 nm (Mark S. Rea, Illuminating Engineering Society of North America, The IESNA Lighting Handbook, Ninth Edition, 2000, pg. 4-1). For example, blue light comprises wavelengths from approximately 450 nm to 495 nm and red light comprises wavelengths from approximately 620 nm to 750 nm. Objects illuminated by incandescent lamps provide pleasing and accurate color rendering information to the human eye. However, continuous-spectrum light sources, such as incandescent and halogen lamps, unfortunately tend to be very inefficient. Much of the radiant energy generated by incandescent lamps is outside of the visible spectrum, e.g., in the infrared and ultra-violet range (Id. at pg. 6-2). For example, only approximately 12.1% of the input energy used to power a 1000-Watt incandescent lamp may result in radiation in the visible spectrum (Id. at pg. 6-11). In addition, the energy consumed in the generation of heat in the filament of an incandescent lamp is essentially wasted since it is not used to produce visible light.
As more steps are being taken in order to reduce energy consumption in the present day and age, the use of high-efficiency light sources is increasing, while the use of low-efficiency light sources (i.e., incandescent lamps, halogen lamps, and other low-efficacy light sources) is decreasing. High-efficiency light sources may comprise, for example, gas discharge lamps (such as compact fluorescent lamps), phosphor-based lamps, high-intensity discharge (HID) lamps, light-emitting diode (LED) light sources, and other types of high-efficacy light sources. A fluorescent lamp comprises, for example, a phosphor-coated glass tube containing mercury vapor and a filament at each end of the lamp. Electrical current is conducted through the filaments to excite the mercury vapor and produce ultraviolet light that then causes phosphor to emit visible light. A much greater percentage of the radiant energy of fluorescent lamps is produced inside the visible spectrum than the radiant energy produced by incandescent lamps. For example, approximately 20.1% of the input energy used to power a typical cool white fluorescent lamp may result in radiation in the visible spectrum (Id. at pg. 6-29).
Alas, a typical high-efficiency light source does not typically provide a continuous spectrum of light output, but rather provides a discrete spectrum of light output (Id. at pp. 6-23, 6-24). FIG. 1A shows the discrete spectrum SPDISC-FLUOR of a compact fluorescent lamp. FIG. 1B shows the discrete spectrum SPDISC-LED of an LED lighting fixture, for example, as manufactured by LLF, Inc. High-efficiency light sources that provide a discrete spectrum of light output are thus called discrete-spectrum light sources. Most of the light produced by a discrete-spectrum light source is concentrated primarily around one or more discrete wavelengths, e.g., around four different wavelengths as shown in FIG. 1A. When there are large ranges between the discrete wavelengths (as shown in FIG. 1A), certain colors are absent from the light spectrum of a discrete-spectrum light source and, thus the human eye receives less color-related information. Objects viewed under a discrete-spectrum light source may not exhibit the full range of colors that would be seen if viewed under a continuous-spectrum light source. When illuminated by a discrete-spectrum light source, some colors may even shift from those that are seen when the object is illuminated with a continuous-spectrum light source. For example, the color of someone's eyes or hair may appear different when viewed outdoors under sunlight or moonlight as compared to when viewed indoors under a fluorescent lamp. As a result, the visual experience, as well as the attitude, behavior, and productivity, of a human may be negatively affected when discrete-spectrum light sources are used.
Recent studies have shown that color affects perception, cognition, and mood of human observers. For example, one particular study completed by the Sauder School of Business at the University of British Columbia suggests that red colors lead to enhanced performance on detail-oriented tasks, while blue colors result in enhanced performance on creative tasks (Ravi Mehta and Rui Zhu, “Blue or Red? Exploring the Effect of Color on Cognitive Task Performances”, Science Magazine, Feb. 5, 2009). As stated in a recent New York Times article, “the color red can make people's work more accurate, and blue can make people more creative” (Pam Belleck, “Reinvent Wheel? Blue Room. Defusing a Bomb? Red Room.”, The New York Times, Feb. 5, 2009). Therefore, since the type of light sources used in a space can affect the colors in the space, the light sources may affect the attitude, behavior, and productivity, of occupants of the space.
Lighting control devices, such as dimmer switches, allow for the control of the amount of power delivered from a power source to a lighting load, such that the intensity of the lighting load may be dimmed. Both high-efficiency and low-efficiency light sources can be dimmed, but the dimming characteristics of these two types of light sources typically differ. A low-efficiency light source can usually be dimmed to very low light output levels, typically below 1% of the maximum light output. However, a high-efficiency light source cannot be typically dimmed to very low output levels.
The color of illumination is characterized by two independent properties: correlated color temperature and color rendering (Illuminating Engineering Society of North America, The IESNA Lighting Handbook, Ninth Edition, 2000, pg. 3-40). Low-efficiency (i.e., continuous-spectrum) light sources and high-efficiency (i.e., discrete-spectrum) light sources typically provide different correlated color temperatures and color rendering indexes as the light sources are dimmed. Correlated color temperature refers to the color appearance of a specific light source (Id. at pg. 3-40). A lower color temperature correlates to a color shift towards the red portion of the color spectrum which creates a warmer effect to the human eye, while higher color temperatures result in blue (or cool) colors (Id.). FIG. 1C is a simplified graph showing examples of a correlated color temperature TCFL of a 26-Watt compact fluorescent lamp (i.e., a high-efficiency light source) and a correlated color temperature TINC of a 100-Watt incandescent lamp (i.e., a low-efficiency light source) with respect to the percentage of the maximum lighting intensity to which the lamps are presently illuminated. The color of the light output of a low-efficiency light source (such as an incandescent lamp or a halogen lamp) typically shifts more towards the red portion of the color spectrum when the low-efficiency light source is dimmed to a low light intensity. This red color shift can invoke feelings of comfort to the human observer, since the reddish tint of illumination is often associated with romantic candlelit dinners and cozy campfires. In contrast, the color of the light output of a high-efficiency light source (such as a compact fluorescent lamp or an LED light source) is normally relatively constant through its dimming range with a slightly blue color shift and thus tends to be perceived as a cooler effect to the eye.
Color rendering represents the ability of a specific light source to reveal the true color of an object, e.g., as compared to a reference light source having the same correlated color temperature (Id. at pg. 3-40). Color rendering is typically characterized in terms of the CIE color rendering index, or CRI (Id.). The color rendering index is a scale used to evaluate the capability of a lamp to replicate colors accurately as compared to a black-body radiator. The greater the CRI, the more closely a lamp source matches a black-body radiator. Typically, low-efficiency light sources, such as incandescent lamps, have high-quality color rendering, and thus, have a CRI of one hundred, whereas some high-efficiency light sources, such as fluorescent lamps, have a CRI of eighty as they do not provide as high-quality color rendering as compared to low-efficiency light sources. Light sources having a high CRI (e.g., greater than 80) allow for improved visual performance and color discrimination (Id. at pp. 3-27, 3-28).
Generally, people have grown accustomed to the dimming performance and operation of low-efficiency light sources. As more people begin using high-efficiency light sources—typically to save energy—they are somewhat dissatisfied with the overall performance of the high-efficiency light sources. Thus, there has been a long-felt need for a light source that combines the advantages, while minimizing the disadvantages, of both low-efficiency (i.e., continuous-spectrum) and high-efficiency (i.e., discrete-spectrum) light sources. It would be desirable to provide a light source that saves energy (like a fluorescent lamp), but still has a broad dimming range and pleasing light color across the dimming range (like an incandescent lamp).